A Practical Spectral Fatigue Analysis Procedure for Topside of Floating Offshore Structures in West Africa

2015 ◽  
Author(s):  
Sungwook Chung ◽  
Minsung Chun ◽  
Kibok Jang ◽  
Youngsuk Suh
Keyword(s):  
West Africa ◽  
Wave Spectrum ◽  
Normal Wave ◽  
Offshore Structures ◽  
Fatigue Analysis ◽  
Analysis Procedure ◽  
Sea State ◽  
Commercial Program ◽  
Wind Sea ◽  
Wrapped Normal

In most offshore projects recently ordered, spectral fatigue analysis is required for design integrity. However, the spectral fatigue analysis is very complicated to implement since it has many variations for parameters and forms of input data, and the classification and commercial software packages are exposing limit to support all those variations. A topside fatigue analysis for a FPSO design in West Africa is one of such a challenging project due to the fact that the specification of the project requires spectral fatigue analysis considering 3-peak Ochi-Hubble wave spectrum, Wrapped normal wave spreading and sea state data with 3 wave components, main swell, secondary swell and wind sea. In this study, a practical spectral fatigue analysis procedure is introduced in order to implement the fatigue analysis using a commercial program SACS. Applying adaptive cosine spreading wave distribution which can approximate Wrapped normal wave spreading is devised for each sea state and the comparison between two wave spreading is carried out. Finally, the proposed methodology is justified by analyzing the characteristics of the sea state in West Africa.

Uncertainties of Wind Sea and Swell Prediction From the Torsethaugen Spectrum

2009 ◽  
Author(s):  
Elzbieta M. Bitner-Gregersen ◽  
Alessandro Toffoli
Keyword(s):  
Continental Shelf ◽  
Wave Spectrum ◽  
Single Mode ◽  
Offshore Structures ◽  
Reasonable Accuracy ◽  
Double Peak ◽  
Design Work ◽  
Practical Applications ◽  
Wind Sea ◽  
Norwegian Continental Shelf

In practical applications, it is usually assumed that the wave spectrum is of a single mode form, and well modelled by a JONSWAP or Pierson-Moskowitz spectrum. This assumption is of a reasonable accuracy for severe sea states. However, moderated and low sea states are often of a combined nature, consisting of both wind-sea and swell and should be characterized by a double peak spectrum. Bimodal seas can have a significant impact on the design and operability of fixed and floating offshore structures as well as LNG terminals. Although several separation procedures for the wave components exist the bimodal Torsethaugen spectrum is probably the only one well established in design work. This spectrum was developed primarily for one location at the Norwegian Continental Shelf (Statfjord Field) but in qualitative terms is expected to be of much broader validity. The present study discusses applicability of the Torsethaugen spectrum for locations outside the Norwegian Continental Shelf and uncertainties related to use of the spectrum.

Coupled Analysis of FPSO and CALM Buoy Offloading System in West Africa

2014 ◽  
Author(s):  
Youwei Kang ◽  
Liping Sun ◽  
Zhuang Kang ◽  
Shuhong Chai
Keyword(s):  
West Africa ◽  
Dynamic Effect ◽  
Extreme Environment ◽  
Coupled Analysis ◽  
Water Displacement ◽  
Sea State ◽  
Small Water ◽  
Water Field ◽  
Wind Sea ◽  
Special Environment

FPSO and CALM (Catenary Anchor Leg Mooring) buoy offloading systems are being extensively employed in water field of West Africa. In this paper we did some research on the response of FPSO and CALM offloading systems under special environment conditions in West Africa. It is found that Swell dominate sea state has much larger effects on wave frequency motions of FPSO than responding results introduced by Wind Sea dominate sea state, and squall and current will introduce large horizontal motions of FPSO, such as surge and sway. As for CALM, swell and wind sea mainly affect its heave and roll motions, respectively. Due to its little wind area, CALM does not show great response to Squall which is the extreme environment condition for FPSO though. By comparing the results from OOL (Oil Offloading Line) dynamic analysis and quasi-static analysis, it could be that OOL dynamic effect has little influence on motions of FPSO. But it can bring large changes of responses of CALM because of its small water displacement which indicates coupled analysis of the whole system is necessary. Besides, investigations on OOL fatigue have also been done, and the results show that CALM motion plays an important role on fatigue damage and life of OOL under different sea states.

FPSO Conference—Estimating Wind-Sea and Swell for FPSO Operability

2005 ◽  
Vol 128 (4) ◽  
pp. 314-321 ◽  
Author(s):  
K. C. Ewans ◽  
L. Vanderschuren ◽  
P. S. Tromans
Keyword(s):  
State Of The Art ◽  
Wave Spectrum ◽  
Sea State ◽  
Art Program ◽  
Motion Responses ◽  
Term Variation ◽  
Directional Wave ◽  
Wind Sea ◽  
One Year

The motion response of an FPSO is sensitive to the relative intensities and directions of the wind-sea and swell components in a sea state, and the operability of the FPSO is a function of the long-term variation in these components. Estimations of the operability are therefore dependent both on how the sea state is described in terms of its constituent wind-sea and swell components, and on how the long-term variability of the sea state is captured. However, there is currently no consensus on how either the sea state or its long-term variability should be described. We investigate these issues by means of a study of the responses of a typical FPSO to the wave fields at a location offshore Namibia and a location off the west coast of New Zealand. We make use of a state-of-the-art program for splitting a directional wave spectrum into wind-sea and swell components, and we examine the effect on the motion responses of allowing the spectra to be split into many swell partitions or constraining the spectral split to a maximum of two partitions, as is often assumed in response calculations. The resulting decompositions are used to examine the effects of swell on hull motions and, hence, to identify methods for generating sea state criteria for operability. In addition, one-year metocean conditions are estimated; these are relevant for analysis of the limits on operations.

On the Uncertainties of Estimating JONSWAP Spectrum Peak Parameters

2018 ◽  
Author(s):  
Kevin Ewans ◽  
Jason McConochie
Keyword(s):  
Time Series ◽  
Spectral Data ◽  
Wave Spectrum ◽  
Nonlinear Effects ◽  
Offshore Structures ◽  
Parameter Estimates ◽  
Sea State ◽  
Floating Structures ◽  
Present Understanding ◽  
Jonswap Spectrum

A spectral description of the wave spectrum is usually required in the design of offshore structures, and a generalised form of the JONSWAP spectrum is often used. The JONSWAP spectrum involves parameters that allow flexibility in the specification of the spectral peak, which is important for the response of both fixed and floating structures, but particularly for the floating structures. The peak of the wave spectrum is also important in nonlinear effects that for example contribute to the probability of a large crest occurring in a sea state time series realisation in a model basin, used to test a design platform. There has been a number of studies focused on the uncertainties of JONSWAP parameter estimates. We review these to establish an overview of the present understanding of the uncertainties, and we undertake further analyses to investigate the sensitivity of the uncertainties to the method of analysis and the types of data typically available for analysis, including both time series and spectral data.

Directional Wave Partitioning and Its Applications to the Structural Analysis of an FPSO

2008 ◽  
Author(s):  
Ruth Lawford ◽  
Jill Bradon ◽  
Thomas Barberon ◽  
Claude Camps ◽  
Richard Jameson
Keyword(s):  
Time Series ◽  
Structural Analysis ◽  
Wave Spectrum ◽  
Wave Spectra ◽  
Offshore Structure ◽  
Sea State ◽  
Environmental Force ◽  
Directional Wave ◽  
Wind Sea ◽  
The Individual

A full characterisation of the individual components of a sea-state is key to enabling the response of an offshore structure to be accurately calculated. This paper discusses the partitioning of a time series of directional wave spectra into wind-sea and swell components with distinct frequency and direction characteristics. Once the wave data have been partitioned, JONSWAP or Pierson-Moskowitz parameters can be fitted to each spectrum using ‘best-fit’ techniques. The result of the partitioning and fitting analyses is a time series of wave parameters defining the wave spectrum for each component of the sea state. A 10-year site specific time series of directional wave spectra has been partitioned in this way and used in the analysis of the Triton FPSO, a turret moored FPSO in the central North Sea. The representation of the directionality and magnitude of each environmental force acting simultaneously on the vessel, allows the relative heading of the vessel to be determined and the mooring and hydrodynamic analyses to be performed. These analyses provided input to a structural analysis of the FPSO, which resulted in an inspection plan for monitoring the effects of the metocean conditions on the unit.

Probabilistic Pseudo-Static Analysis of Risers

1990 ◽  
Vol 112 (1) ◽  
pp. 58-64 ◽  
Author(s):  
Suhail Ahmad ◽  
T. K. Datta
Keyword(s):  
Static Analysis ◽  
Static Loading ◽  
Offshore Structures ◽  
Analysis Procedure ◽  
Simulation Studies ◽  
Static Response ◽  
Sea State ◽  
Marine Risers ◽  
Time Histories ◽  
Water Particle Kinematics

Under many sea states, the marine risers behave in a quasi-static manner. The statistics of the response of the marine risers to random sea states producing quasi-static loading is determined using the spectral description of the sea states. The analysis procedure is similar to that proposed by Burrows [2] for offshore structures. The distribution of the response obtained by the proposed method is compared with those derived from the simulation studies in which ensemble of response time histories is computed from artificially generated histories of water particle kinematics for a given sea state. The results of the study indicate that the distribution of the quasi-static response can be adequately modeled by Pierson-Holmes [4] distribution.

Response Sensitivity to Swell Spectra Off West Africa

2013 ◽  
Author(s):  
Kevin Ewans ◽  
George Z. Forristall ◽  
Marc Prevosto ◽  
Michel Olagnon
Keyword(s):  
West Africa ◽  
Earthquake Engineering ◽  
Extreme Values ◽  
Frequency Function ◽  
System Response ◽  
Data Types ◽  
Natural Period ◽  
Wind Sea ◽  
Wrapped Normal ◽  
Floating Systems

Both hindcast and measured wave data were used to establish various descriptions of the swell spectra off West Africa in the West Africa Swell JIP (WASP). The effect on system responses of the various data types and swell spectral descriptions has been estimated. The response of single degree of freedom oscillators to wave spectra can be used to produce maximum responses as a function of natural period and damping, analogous to the approach used in earthquake engineering. The results can be used to gain insight into the sensitivity of floating systems to the swell characteristics of the location and also to the differences between hindcast and measured data sets. Extreme values of system response can be calculated and used to estimate design spectra, by scaling the measured or hindcast spectrum that produced the largest response. The sensitivity of the responses of floating systems to various simplifications of the hindcast spectra was evaluated by calculating the responses of an FPSO and CALM buoy for a location off Nigeria and southern Angola. It was found that there is little loss of accuracy if the hindcast spectra are decomposed into a maximum of two partitions, one of which may be a wind-sea, and the swell partitions are described with a lognormal function, the wind-sea partition with a JONSWAP-Glenn frequency function, and the directional spreading with a wrapped normal function, together with parametric forms for the widths of the lognormal and wrapped normal functions. As a consequence it is concluded that the parametric forms for the swell frequency and direction spreading can be used in design.

scholarly journals Investigation on Spectrum Estimation Methods for Bimodal Sea State Conditions

Sensors ◽  
2021 ◽  
Vol 21 (9) ◽  
pp. 2995
Author(s):  
Giovanni Battista Rossi ◽  
Francesco Crenna ◽  
Marta Berardengo ◽  
Vincenzo Piscopo ◽  
Antonio Scamardella
Keyword(s):  
Weather Forecasting ◽  
Wave Spectrum ◽  
Least Square ◽  
Estimation Methods ◽  
Random Wave ◽  
Reconstruction Method ◽  
Spectrum Estimation ◽  
Sea State ◽  
Benchmark Study ◽  
Wind Sea

The reliable monitoring of sea state parameters is a key factor for weather forecasting, as well as for ensuring the safety and navigation of ships. In the current analysis, two spectrum estimation techniques, based on the Welch and Thomson methods, were applied to a set of random wave signals generated from a theoretical wave spectrum obtained by combining wind sea and swell components with the same prevailing direction but different combinations of significant wave heights, peak periods, and peak enhancement factors. A wide benchmark study was performed to systematically apply and compare the two spectrum estimation methods. In this respect, different combinations of wind sea spectra, corresponding to four grades of the Douglas Scale, were combined with three swell spectra corresponding to different swell categories. The main aim of the benchmark study was to systematically investigate the effectiveness of the Welch and Thomson methods in terms of spectrum restitution and the assessment of sea state parameters. The spectrum estimation methods were applied to random wave signals with different durations, namely 600 s (short) and 3600 s (long), to investigate how the record length affected the assembled sea state parameters, which, in turn, were assessed by the nonlinear least square method. Finally, based on the main outcomes of the benchmark study, some suggestions are provided to select the most suitable spectrum reconstruction method and increase the effectiveness of the assembled sea state parameters.

Feasibility of Using Hindcast Data for Fatigue Assessment of Permanently Moored Offshore Units in West-Africa

2020 ◽  
Author(s):  
Remco Hageman ◽  
Pieter Aalberts ◽  
Didier L’Hostis ◽  
Alain Ledoux
Keyword(s):  
West Africa ◽  
Statistical Data ◽  
Wave Spectrum ◽  
Accurate Method ◽  
Wave Loads ◽  
Floating Structure ◽  
Statistical Parameters ◽  
Sea State ◽  
Multiple Wave ◽  
Fatigue Assessment

Abstract To register the wave loads acting on offshore floating structures, wave buoys have been the generally accepted as the most accurate method. However, over the last couple of years, hindcast data has become increasingly accurate. The availability of this data allows assessment of the response of offshore floating structure without the need of local wave buoys. The accuracy when using hindcast data to assess fatigue accumulation on FPSO hulls is assessed in this paper. For this analysis, calculations have been executed for multiple spread-moored production units in the West-Africa region. The purposely deployed wave buoys provide a full wave spectrum including multiple wave components and directional variation of the wave energy. This data is used to derive statistical data description of the sea state. In this paper, we consider the use of WaveWatch III, ERA-5 and Copernicus hindcast models. These are all models which provide sea state descriptions on a global scale. The statistical data from the hindcast models is compared against the reference data provided by the wave buoys. Overall, the three models provide decent results in this mild environment. However, it is found that Copernicus provides more accurate estimates for the larger wave heights. Fatigue analyses have been executed. The full spectral data provided by the wave buoys is used to conduct a spectral fatigue assessment. The hindcast models generally provide more limited details of the sea state, information such as spectral shapes and directional spreading may be missing. Understanding the influence of these missing characteristics is vital for reliable long term fatigue assessment. Multiple fatigue analyses have been executed to examine the sensitivity to these missing characteristics. It has been shown that the multi-modality of the sea state and spectral shape are the most important parameters driving the deviation between the fatigue assessment based on hindcast data and wave buoy data. These differences can accumulate to a factor of 2 on lifetime consumption. The influence of the accuracy of the statistical parameters provided by the hindcast models and wave spreading is considerably less with a typical contribution of around 30% on lifetime consumption.

scholarly journals An Analytical Method for Dynamic Wave-Related Errors of Interferometric SAR Ocean Altimetry under Multiple Sea States

2021 ◽  
Vol 13 (5) ◽  
pp. 986
Author(s):  
Yao Chen ◽  
Mo Huang ◽  
Yuanyuan Zhang ◽  
Changyuan Wang ◽  
Tao Duan
Keyword(s):  
Electromagnetic Scattering ◽  
Analytical Method ◽  
Wave Spectrum ◽  
High Accuracy ◽  
Sea State ◽  
Error Budget ◽  
Ocean Topography ◽  
Mean Square Errors ◽  
Dynamic Wave ◽  
Wide Swath

The spaceborne interferometric synthetic aperture radar (InSAR) is expected to measure the sea surface height (SSH) with high accuracy over a wide swath. Since centimeter-level accuracy is required to monitor the ocean sub-mesoscale dynamics, the high accuracy implies that the altimetric errors should be totally understood and strictly controlled. However, for the dynamic waves, they move randomly all the time, and this will lead to significant altimetric errors. This study proposes an analytical method for the dynamic wave-related errors of InSAR SSH measurement based on the wave spectrum and electromagnetic scattering model. Additionally, the mechanisms of the dynamic wave-related errors of InSAR altimetry are analyzed, and the detailed numerical model is derived. The proposed analytical method is validated with NASA’s Surface Water and Ocean Topography (SWOT) project error budget, and the Root-Mean-Square Errors (RMSEs) are in good agreement (0.2486 and 0.2470 cm on a 0.5 km2 grid, respectively). Instead of analysis for a typical project, the proposed method can be applied to different radar parameters under multiple sea states. The RMSEs of Ka-band under low sea state, moderate sea state, and high sea state are 0.2670, 1.3154, and 6.6361 cm, respectively. Moreover, the RMSEs of X-band and Ku-band are also simulated and presented. The experimental results demonstrate that the dynamic wave-related errors of InSAR altimetry are not sensitive to the frequencies but are sensitive to the sea states. The error compensation method is necessary for moderate and higher sea states for centimetric accuracy requirements. This can provide feasible suggestions on system design and error budget for the future interferometric wide-swath altimeter.

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